1. Field of the Invention
The present invention relates to ram air canopy parachutes and more particularly a ram air parachute canopy with improved tensioning to maintain a desired shape during flight.
2. Discussion of Related Art
Parachutes have evolved over the years into highly sophisticated systems, and often include features that improve the safety, maneuverability, and overall reliability of the parachutes. Initially, parachutes included a round canopy. A skydiver was connected via a harness/container system to the canopy by suspension lines disposed around the periphery of the canopy. Such parachutes severely lacked control. The user was driven about by winds without any mechanism for altering direction. Furthermore, such parachutes had a single descent rate based upon the size of the canopy and the weight of the parachutist.
In the mid-1960's the parasol canopy was invented. Since then, variations of the parasol canopy have replaced round canopies for most applications, particularly for aeronautics and the sport industry. The parasol canopy, also known as a ram air canopy, is formed of two layers of material—a top skin and a bottom skin. The skins may have different shapes but are commonly rectangular or elliptical. The two layers are separated by vertical ribs to form cells. The top and bottom skins are separated at the lower front of the canopy to form inlets. During descent, air enters the cells of the canopy through the inlets. The vertical ribs are shaped to maintain the canopy in the form of an airfoil when filled with air. Suspension lines are attached along at least some of the ribs to maintain the orientation of the canopy relative to the ground. The canopy of the ram air parachute functions a wing to provide lift and forward motion. Guidelines operated by the user allow deformation of the canopy to control direction and speed. Ram air parachutes have a high degree of maneuverability.
The shape of the canopy of a ram air parachute during flight is affected by the air passing thorough and around the canopy. Under canopies of conventional design, the leading edge or nose of the ram air parachute is deformed during flight. Since the skins and ribs are formed of highly flexible materials, they provide little structure for maintaining the shape of the canopy. The shape is provided by the internal pressurization caused by air entering the inlets. However, with forward motion, the head-on wind overcomes the internal pressurization of the canopy, and deforms the nose of the canopy. The deformation impairs the aerodynamics of the parachute making the parachute fly less efficiently. Therefore, a need exists for a canopy for a ram air parachute which reduces deformations of the nose.
Paragliders and powered parachutes, which operate with similar designs to ram air parachute canopies, overcome the deformation problem by including “stiffeners” in the nose of the canopy. Typically, the stiffeners are plastic or mylar sheets sewn on the vertical ribs of the canopy., typically sewn into the nose of the canopy on the vertical ribs. The stiffeners reinforce the nose of the canopy and help maintain its shape. The stiffeners also function to keep open the inlets of the canopy when not inflated to aid in the launching of para-gliders and powered parachutes. However, the stiff plastic or mylar used in paragliders and powered parachutes is not applicable to skydiving or other freefall deployable parachutes. A deployable system must be packed into a small space and must open efficiently. The stiffeners cannot be crushed for packing and cannot be arranged for effective deployment. When stiffeners become crushed, they remain creased or bent and create additional deformation of the nose of the canopy, which hinders proper operation of the parachute. Packing of such paraglider or powered parachute is not possible due to the stiffeners.
During flight, when the canopy is inflated, the loading and pressure distribution result in an airfoil shape which is not a smooth shape. Since canopies are flexible structures, they tend to distort based upon tensions, stresses, and airflows. Accordingly, as the parachute glides through the air, the pressurization of the canopy causes the canopy to stretch, particularly at the nose of the canopy. The stretching of the canopy causes the wing-shape of the parachute to distort such that the aerodynamics are compromised, thus resulting in efficiencies. Moreover, the performance of the parachute is decreased over time due to the span-wide stretching of the canopy.
Although a cell is modeled as having a basically rectangular cross section, when inflated, the shape distorts towards round. Typically, in a ram air parachute, suspension lines are attached to every other rib, thus creating loaded ribs (i.e., ribs to which suspension lines are attached) and non-loaded ribs (i.e., ribs which do not have suspension lines attached thereto). The different stresses on the loaded and non-loaded ribs also distorts the cell shape. FIG. 1 illustrates a cross section of a portion of a typical ram air parachute canopy 500 during flight. FIG. 1 shows four cells 501, 502, 503, 504 with three loaded ribs 510, 511, 512 and two non-loaded ribs 521, 522. Suspension lines 541, 542, 543 are attached to the loaded ribs 510, 511, 512. The top skin 530 and bottom skin 531 tend to arc between the ribs during inflation. Also, the non-loaded ribs 521, 522 tend to be higher than the loaded ribs 510, 511, 512, which provides a distortion along the width of the canopy. The distortion is aerodynamically undesirable and adversely affects performance of the canopy.
In order to keep the loaded and non-loaded ribs level and to improve upon the aerodynamics of the canopy, cross-bracing between ribs has been added to the canopy. U.S. Pat. No. 4,930,728 illustrates such a design. FIG. 2 illustrates a design with cross-bracing, called a “tri-cell” configuration. In the tn-cell configuration, there are two non-loaded ribs 720, 721 between the loaded ribs 710, 711. Cross braces 751, 752 connect the bottom of each loaded rib 710, 711 with the top of a non-loaded rib 720, 721. Accordingly, the cross brace angle is not too acute. The resulting canopy has three, small, span-wise distortions between loaded ribs instead of two large distortions as in a canopy without cross bracing. Since the cell between non-loaded ribs has no cross-bracing, this design is less rigid than a fully cross-braced canopy.
Cross-bracing suffers from a number of drawbacks. The cross-bracing results in a complicated construction, high manufacturing costs, and increased packing volume. The tri-cell design has a packing volume approximately 25% larger than a non-cross braced design for a nine cell canopy. Furthermore, the rigidness induced by the cross-bracing creates high opening forces. Typically, large cross porting is used on all of the cells, which causes the canopy to essentially inflate all at once. The opening forces can be so severe that they can jar the jumper's body causing discomfort and even serious injuries. Although designers have implemented “formed” noses, larger sliders, moved bridal attachment points and modified line trims to try to soften the openings of such cross-braced canopies, it has generally yielded only limited improvement to the point where the openings are marginally acceptable.
Accordingly, a need exists for a canopy design which reduces distortion without having to use cross braces.